We confirm the reality of a reversal of the sign of the Faraday rotation
measure in the Galactic plane in Cygnus (Lazio et al. 1990), possibly
associated with the Cygnus OB1 association. The rotation measure changes
by several hundred rad m-2 over an angular scale of
2°-5°. We show that a simple model of an expanding plasma shell
with an enhanced density and magnetic field can account for the
magnitude and angular scale of this feature. This model is consistent
with observations of Hα emission as well as other observations in
this part of sky. We suggest that this structure is physically
associated with a superbubble produced by the Cygnus OB1 association.